The present invention relates generally to piston pumps and, in particular, to a novel piston pump with a novel pump inlet check valve.
Piston pumps are well known. They are positive displacement pumps that typically consist of a pump housing with one or more cylinders contained therein, a respective piston received in each of the cylinders, a respective cylinder heads closing one end of the cylinders, a drive for reciprocating the pistons in the cylinders (e.g., an electric motor and a camshaft), and fluid passageways for routing the working fluid into and out of the pump. The working fluid is introduced into the cylinders, typically through respective inlet check valves, pressurized in the cylinders by the movement of the pistons between the pistons and the respective cylinder head, and urged out of the pump through the fluid passageways, typically through respective outlet check valves. The pistons typically utilize a return spring to urge the piston into contact with the rotating camshaft. The inlet and outlet check valves may also utilize return springs to aid in closing these valves when the pressure drops on the respective upstream side.
During the pump operation, the piston moves away from the cylinder head, reducing pressure in the cylinder, closing the outlet check valve and opening the inlet check valve, and drawing fluid into the cylinder. When the piston is subsequently driven toward the cylinder head, the pressure in the cylinder increases, the inlet check valve closes, and the trapped fluid is pressurized in the cylinder as the piston continues its upward motion. This motion of the piston toward the cylinder head is termed the compression stroke. The outlet check valve opens and allows the pressurized fluid to be delivered to the downstream fluid passageways. The outlet check valve remains open until the pressure in the cylinder decreases, typically when the piston begins away from the cylinder head again. The outlet check valve then closes, the inlet check valve opens, and the cycle is repeated. This motion of the piston away from the cylinder head is termed the suction stroke or intake stroke.
Prior art piston pumps often have been characterized by large unswept volume. Unswept volume is defined as that volume in the cylinder that contains fluid that is compressed when the piston moves from bottom dead center (BDC) to top dead center (TDC), minus the uncompressed volume of fluid that is displaced as the piston moves from BDC to TDC. Unswept volume thus represents the volume of fluid that the pump works to bring to a high pressure, but which remains in the cylinder. The pump has to perform work on, or compress, a set amount of fluid volume with every piston compression stroke, with only a smaller amount of the volume of fluid compressed actually being delivered to the fluid system beyond the outlet check valve. It is therefore desirable to minimize unswept volume.
Furthermore, many prior art pumps were designed with a coil return spring for the piston or the inlet check valve disposed in cylinder between the cylinder head and the piston, thereby limiting how closely the piston could approach the cylinder head, and increasing the unswept volume. This decreases the pump""s efficiency.
As noted above, prior art piston pumps utilized an inlet check valve that allowed fluid to flow ahead of the piston on the suction stroke and closed at the bottom of the stroke, usually with the aid of a return spring. It is known in the prior art to limit the movement of the movable element of a check valve so that the movable element cannot get too far from the valve seat so that the movable element reseats more readily when the piston of the associated pump starts on the compression stroke. Prior art pumps with construction optimized at higher temperatures will not operate as efficiently at a low temperature. Pumps thus constructed have structures to keep the movable element of the pump""s inlet check valve close to the respective seat to minimize backflow of the hot and relatively low viscosity fluid. However, because the ball is not allowed to move far enough off the seat, the inlet check valve does not allow low temperature, relatively high viscosity fluid to pass freely in the direction of pumping. Conversely, prior art pumps with inlet check valves constructed to work well at low temperature will be less efficient at higher temperature. Such pumps are constructed to allow the movable element to move far from the seat to minimize heat loss while pumping relatively viscous cold fluid. When warm, the movable element is off the seat for too long during the compression stroke of the pump piston and allows excessive fluid to return through the inlet valve instead of being pumped out.
A piston pump includes a pump housing defining a cavity therein. A cylinder has open first and second ends and is attached to the pump housing within the cavity of the pump housing. A longitudinal bore has an inlet at the first end of the cylinder and an outlet at the second end of the cylinder. An outlet check valve seat is defined about the outlet of the second end of the cylinder. A cylinder head is attached to the cylinder and to the pump housing. The cylinder head encloses the open second end of the cylinder. A second passage way is formed within the cylinder head and has an inlet and an outlet extending from the cylinder head to an aperture in the pump housing. A piston is slidably received in the open end of the cylinder. The piston has a first passageway formed therein. The first passageway has an inlet end in fluid communication with the cavity of the pump housing, and an outlet end in fluid communication with a pumping chamber defined in the pump housing. As the piston moves, the volume of the pumping chamber is varied. A first spring member is attached to an exterior portion of the piston and to the cylinder for retaining the piston in the cylinder. A first check valve member is disposed in the outlet end of the first passageway of the piston to allow fluid to flow only from the inlet end to the outlet end of the first passageway of the piston. In a preferred embodiment, a shoulder defined at the outlet end of the first passageway of the piston defines a piston valve seat, and the first check valve member is embodied as a ball, which is retained near the piston valve seat by a generally planar retaining element. In the preferred embodiment, the retaining element is generally cup-shaped and has a plurality of apertures formed therethrough for the passage of fluid.
In another preferred embodiment, the retaining element includes a disk spring for varying the distance that the ball may move off the piston valve seat.
In another preferred embodiment, the retaining element has a temperature sensitive design in which the distance that the retaining element permits the ball to move off the piston valve seat varies according to the temperature of the fluid passing through the pump.
In another preferred embodiment, the first check valve member includes a movable valve element which is urged toward to an associated piston valve seat by a spring fastened to both the movable valve element and the piston, with the seat being pressed into the outlet end of the first passageway of the piston.
In another preferred embodiment, the first check valve member includes a flat disk selectively sealing against the piston about the outlet of the first passageway. The pump also includes an outlet check valve permitting pressurized fluid to flow from the pumping chamber to the outlet of the piston pump. In various preferred embodiments the outlet check valve can be embodied as a ball check valve or a check valve having a generally flat disk shape.
Various objects and advantages of this invention will become readily apparent to those skilled in the art from the following detailed description of the preferred embodiment, when considered in light of the accompanying drawings.